This invention relates in general to monitoring of radiation sources, and in particular to the use of frequency lockers for monitoring frequency shift of radiation sources. The invention is particularly useful for enhancing performance of wavelength division multiplexing in fiber optics communication.
As fiber optics matures and gradually replaces microwave links in telecommunication industry, there is great need for increasing the capacity of these optical links. One way to achieve higher capacity is to increase bit rate of the link. This is called time division multiplexing, where multi-Giga bit of data is transmitted across this media. While time division multiplexing is adequate for moderate speed links, high capacity links have increasingly turned to wavelength division multiplexing (WDM), where multiple wavelength channels are simultaneously used to transmit data across the fiber.
WDM based systems have evolved rapidly from early two channel systems to the current 8 channel system. International Telecommunication Union (ITU) has even proposed a 45 channel system utilizing wavelength range from 1533 to 1565 nm with channel spacing of 100 GHz (about 0.8 nm). This system, referred to herein as the ITU system, is pushing the state-of-art of the various fiber optics components. It challenges optical component manufacturers to provide ultra-narrow bandwidth filters, transmitting lasers with highly stable frequencies, and optical components with high bandwidth capability etc. This invention relates to the enhancement of laser stability attained via the application of Fabry-Perot interference filter in laser frequency monitoring and control.
The currently deployed laser systems are based on Distributed feedback (DFB) lasers operating in 1550 nm range. These lasers can be tuned in frequency by heating or cooling the laser using Thermal electric cooler. However, there is up until now, no frequency reference at reasonable cost for these lasers to lock on to establish less than 0.1 nm wavelength shift over the field operating temperature range from -20 to 85.degree. C. This invention describes the many implementations of this stable reference filter based on Fabry-Perot interferometer principle.
It is, therefore, desirable to provide a frequency reference at reasonable cost for lasers in order to maintain the laser at predetermined frequencies.